Electric discharge device



May 2, 1939- I L. E. FLORY ET AL 2,156,625

ELECTRI C DISCHARGE DEVICE Filed March 50, 1935 www E TTOBNEL/ PatentedMay 2, 1939 UNITED STATES PATENT OFFICE 2,156,625 ELEc'rmo DISCHARGEnEvrcE Ware Application March 30, 1935, Serial No. 13,806

4, Claims.

stream, such, for example, as is emitted from a y thermionic cathode orfrom a photosensitive sur- -face exposed to light, is accomplishedthrough utilization of the phenomenon of secondary emission.

If an electrode is subjected to electron Abom, bardment,it will emitsecondary electrons. The ratio of the number of secondary electrons tothe number of primary electrons depends, in part, upon the character ofthe bombarded surface and upon the potential difference between thesurface y and the source of the electrons. This ratio can be madeconsiderably greater than unity. For example, a ratio of five or moresecondary electrons to one impinging electron is readily obtainable withmetallic surfaces treated in known ways and subjected tovdischarges atpotentials of 300 to 400 volts.

Ifthe secondary electron current, in turn,is caused to impinge withsuillcient velocity upon a further electrode with a suitably treatedsurface, the ratio of secondaryV emission from the second multiplyingelectrode may also be greater than unity. Hence, one is able to obtainwith "n multiplying electrodes in cascade, for example, an amplificationof the original or pri-- mary electron current equivalent to theampliiication per electrode raised tothe nth power. A million-foldamplification may be obtained in a single device.

Prior art devices, wherein the phenomenon of secondary emission isutilized for amplification,

have proved to be unreliable and ineflicient, principally because ofincomplete and uncertain control and utilization of the secondaryelectron streams. For example, in the operation of a de- 40 viceconstructed according to French Patent 582,428 (Dapsence et aL), thesame electrostatic fields are utilized for electron acceleration and fordetermining the paths described by the primary and secondary electrons.It has been found 46 that, in general, it is verydiilicult to controlthe paths of the electrons in a tube of that type in such manner as tocause all of the electrons from any one source to fall upon the desiredtarget. Another disadvantage of previous electron mul- 50 tipliers, oftypes exemplified by the French patent,

resides in the fact that the iield in the neighborhood of eachmultiplying electrode, which serves to draw off the secondary electrons,mustk necessarily be weak. Because of the small magnitude 55l of theeld, itis impossible to draw large cur- (Cl. 25o-166) rents from thoseelectrodes. An attempt to overcome this defect by increasing thepotential of the succeeding electrode, with the object of increasing theaccelerating field in the neighborhood of the preceding multiplyingelectrode, generally re- 5 sults in a decreased gain since some of theprimary eletrons which would normally strike the preceding multiplyingelectrode are drawn past it to `one of the following electrodes. Asa-consequence, because of the space-charge limitation 10 of current, itis impossible to maintain linearity between the input excitation or thecurrent from the primary source, and the output current, except forextremely we ak output currents.

It is, accordingly, an object of our invention to l5 provide an electricdischarge device, utilizing secondary electron emission, wherein maximumgain per stage is obtained and linearity may be obtained between inputexcitation and output current.

Another object of our invention is to provide an 20 amplifier orelectron multiplier of the secondary electron emission type in which thesecondary electron Stream from each emitter is concentrated and isdirected accurately to the desired 25 target, and interference betweenthe various electron streams is minimized.

Another object of our invention is to provide an amplier or electronmultiplier wherein there is practically no loss of secondary electronsand in 30 which the best conditions for amplification, or other desiredresults, can easily be obtained by external potential adjustments.

Another object of our invention is to provide an amplifier or electronmultiplier that is eiicient 35 and reliable in operation and in whichthe ampliiication obtainable is very great as compared with theamplication obtainable with a thermionic amplier of usual type.

Another object of our invention is to provide a 40 device of the typedescribed that may be used for substantially any purpose for whichthermionic tubes of present types are used, Such, for example, as anamplier, a demodulator, an oscillator, a

combined oscillator and modulator, etc.

Another object of our invention is to provide a combined photocell andamplier that shall be responsive to the very highest frequenciesencountered in television transmitting apparatus.

A still further and more specific object of our invention isto 'providea device of the type described that lends itself readily to massproduction methods.

The foregoing objects and other objects ancillary thereto we prefer toaccomplish by/the provision of separate and independentilelds for theproduction of secondary electrons from one electrode andthe focusing ofthe electrons upon the next succeeding electrode. In particular, in apreferred embodiment of our invention we utilize an electrostatic fieldto cause electrons emitted from a primary source, such as a cathode,either photoelectric or thermionic in character, to be acceleratedtoward an electrode having a surface capable of secondary emission. Thesecondary electrons thus produced are directed against an outputelectrode or another similar surface at a still higher positivepotential whereat further secondary electrons are produced. In each caseelectromagnetic focusing means are made use of for the purpose offocusing Vand directing the primary electrons toward the rst secondaryemitter and the secondary electrons from the rst emitter toward the nextsucceeding electrode. 'I'his process may be repeated a number of timeswithin the same container if desired, there being no limit to the numberof secondary emitters interposed between the cathode and the outputelectrode.

The novel features which we believe to be characteristic of ourinvention are set forth with particularity in the appended claims. "Ourinvention itself, however, both as to its organization and method ofoperation will best be understood by reference to the followingdescription taken in connection with the accompanying drawing, in which:Y

Figure 1 is a view in perspective of an embodiment of our invention, and

Fig. 2 is a diagrammatic view exemplifying the circuit connectionsutilized when our device is employed as an amplifier.

Referring now to Fig. 1` of the drawing, an electron. discharge deviceconstructed according to our invention may be constituted by a Y-shapeevacuated container I. Within the container adjacent to the extreme endof one of the arms 3 is mounted a photosensitive cathode 5 and an outputelectrode 'l is disposed Within the container adjacent the end of theother arm 9. An electrode II capable of emitting secondary electrons isdisposed within the stein I3 of the container in such position that itis accessible to electrons emitted from the photosensitive cathode 5 andis also visible from the output elec-` trode 1.

It is to be understood, of course, although we have chosen aphotosensitive electrode for purposes of illustration, that it may bereplaced by any other suitably controllable electron source and,alternatively, that electrons may be introduced into the device from anoutside source.l

It is also to be borne in mind that we are not limited to the singlestage amplification shown in Fig. 1 since, merely for purposes ofexplanation, we have chosen the simplest possible form of our invention.

For purposes of convenience the electrode II mounted in the stem of thecontainer will be referred to as a multiplying electrode.

Electrons emitted from the photosensitive electrode 5 or cathode leavethe surface in random directions. For the purpose of focusing them uponthe multiplying electrode II we provide an electromagnetic coil I5 whichencircles the arm 3 of the container between the cathode 5 and the saidmultiplying electrode. Also, for the purpose of focusing and directing,upon the output electrode 1, secondary electrons from the multiplyingelectrode, a similar electromagnetic coil I1 encircles the arm of thecontainer between the said output electrode and the multiplyingelectrode.

An accelerating electrode I9, preferably in the form of a grid, isinterposed between the cathode and the multiplying electrode and asimilar accelerating electrode 2l is interposed between the multiplyingelectrode and the output electrode. These accelerating electrodes alsofunction to shield the input electrode from the output electrode. Theaccelerating electrode between the multiplying electrode and the outputelectrode also has an additional function of removing secondaryelectrons from the vicinity of the multiplying electrode and of urgingthem into the focusing field set up by the coil adjacent thereto.

In the manufacture of the device illustrated, the glass envelope wasfirst fabricated in the form shown, the ends of the arms and the end ofthe stem being left open to permit the sealing therein of the respectiveelectrode-supporting presses. The accelerating electrodes, which werepreferablyy made of nickel, were next sealed in place, and the input,multiplying and output electrodes, each of which had previously beenprovided with an individual press, were next sealed in the open ends.'I'he input and multiplying electrodes were made of silver,substantially ten-thousandths of an inch thick and the output electrodewas made of tantalum or an analogous metal, approximatelyflve-thousandths 0f an inch thick.

After the electrode presses were in place, the tube was sealed to a highvacuum system by means of a tubulation (not shown), through which thetube could be evacuated. An appendage (not shown) containing pellets ofa caesium compound, such as caesium chromate, and a reducing agent, suchas aluminum powder or silicon powder, was sealed onto the tube by meansof another tubulation (not shown) through which the caesium from thepellets could be admitted to the main body of the tube.

The tube was then baked at 450 centrigrade, being evacuated at the sametime. 'I'he bake continued for approximately thirty minutes after theoven reached the final temperature. After the baking, the tube wascooled and a small amount of pure oxygen was introduced into it at apressure of approximately 1 mm. of mercury. The cathode and multiplyingelectrodes were next oxidized by passing an electrical'discharge fromthese elements to some other element in the tube until the electrodesurfaces acquired a bluishgreen tinge. The oxygen was then pumped outand the pellets of caesium compound and reducer were heated sufiicientlyto start the reaction which yields metallic caesium. The metalliccaesium was driven, by means of heating the appendage, into the mainbody of the tube. The tube was once more baked at 200 centrigrade forapproximately ten minutes and allowed to cool. 'Ihe caesium appendagewas then sealed off the tube and the tube sealed off the vacuum system.

It might be thought that the caesium would be deposited upon theaccelerating and output electrodes. Such is undoubtedly the case, but,by reason of the greater afiinty of caesium for an oxide, particularly'silver oxide, when the tube was heated in the final heating, most ofthecaesium was driven oiilr the other elements and was taken up by I theoxidized silver electrodes. 'I'he caesium forms a chemical compound withthe silver oxide which is reasonably stable, al-

though the actual chemical reaction that takes place ls notdeflnitelyand accurately known.

The operation oi' our improved electron discharge device as anampliflerof fluctuating light,

, such as that provided by a moving sound track on4 a talking motionpicture film or the like, is exemplified by Fig. 2 of the drawing. Whenutilized as such an amplifier the photosensitive cathode 5 may beconnected to the negative terminal of a potential divider 23 connectedacross a source 25 of unidirectional lpotential .and the firstaccelerating electrode, the multiplying electrode, the secondaccelerating electrode and the output electrode, preferably, areconnected to points on the potential divider successively more positive.'I'he relative potentials shown in the drawing are to be construedsolely as illustrative, and our invention is not to be limited thereby.

If desired, the primary winding 21 of an output transformer 29 may beconnected between the output electrode and the potential source. Thesecondary winding 3l of the transformer may be connected rdirectly inseries, or otherwise, with the voice coil 33 of a loudspeaker 35 or theterminals of the said secondary may be connected to the input terminalsof any other indicating device or to an amplifier.

The several focusing coils may be provided with unidirectional potentialfrom a battery 31 or the like. In the drawing these coils areexempliiied as being connected in parallel to the potential source, apotential divider 39 and a plurality of contact devices Il and 43 beingutilized for the purpose of individually controlling the magnitude oftheseveral iield currents. The polarity of the coils appears to beimmaterial.

Any source of uctuating light may be utilized to energize thephotosensitive cathode. -Such source is exemplified by a movingphotographic sound record 45 which is transilluminated by anincandescent light 41 and the image of which is thrown upon the cathodeby a lsuitable optical system 49.

When light, modulated by the film, strikes the cathode 5` photoelectronsare emitted therefrom in random directions under the iniiuence of thepositive potential applied to the first accelerating electrode I9. Inorder that substantially all of these electrons may be utilized, themagnetic field set up by the first coil I5 (reading from right to leftin the drawing) is so adjusted that they are concentrated and brought tofoci upon the multiplying electrode. As the electrons impinge upon themultiplying electrode they cause the liberation of secondary electronswhich are removed from the vicinity of the said multiplying electrode bythe second accelerating electrode 2Iand brought under the influence ofthe electrostatic iield between the said accelerating electrode and theoutput electrode 1. concurrently, the magnetic field set up by the coilI1 adjacent to the output electrode functions as an electron lens toconcentrate the secondary electrons and to direct them-upon the outputelectrode. The coil -I1 isnot absolutely necessary in the device i1-lustrated, since the accelerating electrode 2|, being maintained at highpositive potential, will remove all secondary electrons emitted from themultiplying electrode. In a multi-.stage device, wherein a secondaccelerating electrode would be substituted for the output electrode,the focusing coil is necessary.

It should also be understood that'we are not limited to the use of aphotosensitive cathode,

since it lies lwithin the scope of our invention to replace it by anysuitable electron source, such as one of the thermionic type, and toprovide one or more grids for the purpose of suitably controlling theelectron emission. yThe manner in which a controllable thermionic sourcecould be mounted in the container in lieu of the photosensitive cathode5 will be `perfectly apparent to those skilled in the art and nonecessity is seen for illustrating it. If further information, however,is desirable, attention is directed toward the copending application,Serial No. 4,049, led January 30, 1935, in the name of Louis Malter, andassigned to RadioCorporation of America.

Our improved device, in addition to its capability of providing anamplified fluctuating output j current in accordance with a uctuatinglight tional to input excitation but which follows some other curve.Non-linearity may be, for example,

obtained by interposing a'n impedance 'device 5| between the potentialdivider and the ilrst multiplying electrode. 'Ihis impedance device maybe a resistor or an inductor having such electrical characteristicsthat, when the current therethrough changes, the potential on themultiplying electrode also changes to alter the gain in proportionthereto. An inductor, of course, would be preferable if a characteristicnon-linear with respect to frequency is desired.

Our improved device, either of the photosensitive cathode or thethermionic cathode type, is capable of generating sustainedoscillations. This may be accomplished through utilization of any of theconventional regeneration circuits which permit of feeding back aportion of the output potential to the input circuit in proper phase. g

Sight should not be lost, also, of -theiact that our device is capableof use as a demodulaton Such purpose may be accomplished by applying thesignal to be demodulated either across the impedance device 5I, shown inFig. 2, or in series therewith. g

Many other uses of our device will be apparent to those skilled in theart, such, for example, its use as a combined oscillator-modulatorthrough the application of modulating potentials to the multiplyingelectrode while the carrier frequency is applied tothe input circuit orwhile the device is in the`self-oscillatory condition. Since it issubstantially impossible to list herein all of the possibilities of ourimproved device, it is to be distinctly understood that our invention isnot to be circumscribed by the examples given.

' By reason of the shape of our improved electric discharge device andthe presence of the electron lenses, electrons from any given source are.prevented from being driven past their preallocated target and arecaused to impinge thereon.

Because of the fact that the acceleration of the electrons is producedthrough the action of an electrostatic field which does not interferewith the focusing action of the electromagnetic electron-lenses, thespace charge limitations of prior devices are avoided. The eciency thusobtained is much greater than where dependence is placed solely uponelectrostatic fields between adjacent electrodes for the purpose of bothaccelerating the electrons and directing them to the secondary emitters.By providing the electron lenses, the impinging electron stream isconcentrated and directed to the emitting surface and at the same timethere may be maintained at the surface an electrostatic potentialgradient which is favorable for removing the emitted electrons withmaximum eillciency. Substantially no interference or interruptionbetween the high velocity impinging electrons and the low velocitysecondary electrons has been observed and it seems highly probable thatthere is no actual interference between the electromagneticconcentrating iields and the electrostatic accelerating iields.

It follows from the foregoing enumerated facts that, in our improveddevice, the output current is not limited by space charges. As a result,the device appears to have no saturation point, the amountvof outputcurrent which it can provide being only dependent upon the amount ofheat which the electrodes can dissipate, upon their resistance todestructive electrostatic forces, the potentials applied to theelectrodes and the adjustment of the focusing action of the electronlenses through suitable control of the potentials applied thereto.

We are aware of many physical modifications of our device and many otherpossible uses thereof that at once will be apparent to those skilled inthe art. Our invention, therefore, is not to be limited except insofaras is necessitated by the prior art and by' the spirit of the appendedclaims.

We claim as our invention:

1. An electric discharge device including an evacuated envelope, anelectron source, a secondary emitter electrode and a collectorelectrodemounted in the order named within said envelope, said emitterelectrode being accessible to electrons from said source and saidcollector electrode being accessible to secondary electrons from saidemitter electrode, an accelerating electrode mounted in the path of theelectrons intermediate said source and said collector electrode andmeans whereby an electromagnetic electron-lens may be established in thepath of the electronsy which are subject to the influence of saidaccelerating electrode whereby said electrons may be concentrated andbrought to foci on the electrode toward which they are directed.

2. The invention as set forth in claim 1 wherein an acceleratingelectrode and means for establishing an electromagnetic electron-lensare provided intermediate said electron source and said emitterelectrode.

3. An electric discharge device including an evacuated envelope, anelectron source, a secondary emitter electrode and a collector electrodemounted in the order named within said envelope, said emitter electrodebeing accessible to electrons from said source and said collectorelectrode being accessible to secondary electrons from said emitterelectrode, means whereby an electromagnetic electron-lens may beestablished intermediate the emitter electrode andthe collectorelectrode for concentrating the secondary electrons and bringing them tofoci upon the collector electrode, and an accelerating electrode in thepath of the secondary electrons for drawing the secondary electronsWithin the inuence of said electromagnetic electron-lens.

4. An electron multiplier comprising a source of electrons, amultiplying electrode capable of emitting secondary electrons, an outputelectrode, leads connected to said source and electrodes whereby anelectrostatic field may be established between the source and themultiplying electrode, and whereby an electrostatic eld may beestablished between the multiplying electrode and the output electrode,means for establishing two electromagnetic fields, each parallelrespectively to the electrostatic fields, and means comprising agrid-like electrode mounted in the path of the electrons for shieldingthe electron source from the output electrode. n

LESLIE E. FLORY. GEORGE A. MORTON.

